Symmetrical components
Symmetrical components
(OP)
Hi all
As I was going through symmetrical components , I came across some question and hope that many of you could answer this.
1. In balanced network , only positive sequence current exists.
2. When there is earth fault in a phase , I assumed that zero sequence flows (considering star connection with earth). Positive , negative and zero sequence network are in series in the faulted phase and are equal in magnitude. Should positive and negative sequence (because of unbalance load in three phase because of fault) flow in other two un-faulted phase ?
3. When there is unbalanced load in all three phases , only positive and negative seq should flow in all three phases ?
I just wanted to know whether the statement two (2) and three (3) are correct or not. If they are wrong , what should be the correct modification.
regards
As I was going through symmetrical components , I came across some question and hope that many of you could answer this.
1. In balanced network , only positive sequence current exists.
2. When there is earth fault in a phase , I assumed that zero sequence flows (considering star connection with earth). Positive , negative and zero sequence network are in series in the faulted phase and are equal in magnitude. Should positive and negative sequence (because of unbalance load in three phase because of fault) flow in other two un-faulted phase ?
3. When there is unbalanced load in all three phases , only positive and negative seq should flow in all three phases ?
I just wanted to know whether the statement two (2) and three (3) are correct or not. If they are wrong , what should be the correct modification.
regards






RE: Symmetrical components
1) Correct.
2) The sequence networks are in series when calculating the earth fault current. The phase currents are calculated from the sequence currents using the (well known?) transformation matrix.
3) There may be zero sequence currents, if the (physical) network is connected to earth in more than one point.
The statement "negative or zero current flows somewhere in the network" means that when the phase currents are transformed to sequence currents, the resulting negative or zero sequence current is not zero. But one must know all phase currents for the transformation.
RE: Symmetrical components
The phase voltages and currents can be expressed as a sum of the sequence components. For example, for the voltage Va of phase a the sum is: Va = Va0 + Va1 + Va2, where the indices 0, 1, and 2 refer to the zero, positive and negative sequence components, and similarly for the other phases. Thus, actually, one can talk about the sequence components of the voltage or current of a given phase. But the point is, these are not generally needed. Only the sequence components of phase a are needed. These are generally called as _the_ sequence components, dropping the subscript a for the phase.
The sequence components of other phases can easily be calculated from the sequence components of phase a (_the_ sequence components), usig the definition of the sequence components. This means that if some sequence component is nonzero, then it is nonzero in all phases.
(Did I get this right this time?)
RE: Symmetrical components
The only thing I would add to what ijl said is that to get the phase voltages for the other phases, you have to shift the phase angles of the positive- and negative-sequence voltages. This is done by the transformation matrix that he mentions.
RE: Symmetrical components
Thanks